iPhone sync to Mac ends with an error " Songs can't be found"

iPhone sync to Mac ends with an alert:

Some items, including “Harlem In Havana”, were not copied to the iPhone “Harvey’s iPhone 15 Pro” because they could not be found.

The error report lists more than 100 songs, same error.

Yet, I know many (maybe all) of these songs are on the iPhone because I've heard them while listening to a random playlist.

Why is this happening, and should I be concerned?

Thanks.

OS Sequoia latest, iOS 18 before 18.5 update. Music library goes back many years.

[Re-Titled by Moderator]